The 1.2 mm emission arises mostly from cold dust ( K)
associated with molecular clouds. Using the local Galactic value of the dust
absorption cross section, corrected for the larger metallicity observed
near M51's center, we derive the gas distribution from this emission.
The derived molecular masses are times smaller than those obtained
from the CO luminosity, using Strong et al.'s Galactic CO to H
conversion factor ( 10), but close to the masses estimated from
CO and from visual extinction.

It may be argued, considering the crude approximations inherent to each method,
that all estimates are encouragingly close. We wish to stress, however, that
the present work points towards an H mass in the inner part of M51 a
factor of 4 lower than usually assumed from the CO integrated line
intensity. A similar conclusion was reached for NGC 891 (Guélin et al. 1995,
A&A 279, L37). It probably also applies to many galaxies and could have major
consequences on their evolution.

Figure 6: The 1.2 mm continuum emission ( --- top) and
the velocity integrated CO line intensity
(
--- bottom)
in the inner part of M51, smoothed to a 15 angular resolution. The
position of selected H regions are indicated on the continuum map
(numbering according to Carranza et al. A&A 1, 479). Contour levels range from
10 to 46 mJy (beam) in steps of 3 mJy.(beam) for the
continuum, and from
5 K.kms to 32.5 K.kms in
steps of 2.5 K.kms for CO. The continuum emission was set to zero
outside the diamond-shaped grey area. Both maps were
observed with the IRAM 30-m telescope; the CO data are taken from
Garcia-Burillo et al. (1993, A&A 274, 123).